My thanks to the two respondents. I am a little slow in replying, having been on holiday. On the CC0‑1.0 versus CC‑BY‑4.0 question, I am relatively ambivalent. But in many cases, CC‑BY‑4.0 material will be mixed in and that license will then necessarily prevail. Interesting to note (as mentioned earlier) that a literature review suggests that those involved in scientific research favor CC0‑1.0 while those dealing with public interest information provision favor CC‑BY‑4.0 due to its improved ability to retain provenance — more here.
An earlier remark by @SimonPoole that the CC‑BY‑4.0 model distinguishes between existing and adapted material may work for prose but it does not work for data in general. The changes involved are normally too fine grain to track in this manner. Moreover @SimonPoole writes:
The above statement is simply incorrect. The OGL‑UK‑3.0 states that CC‑BY‑4.0 is inbound compatible, hence the reverse cannot be true unless both licenses are legally equivalent — which they are not. Just one seemingly trivial condition will be sufficient to show why: the OGL‑UK‑3.0 has a choice of law provision and the CC‑BY‑4.0 does not. Indeed that one seemingly minor governing law clause is enough to prevent reverse compatibility. Arguments about how lax or otherwise different licenses might be are simply inadequate in this context — indeed the small details are material.
So that means that the OGL‑UK‑3.0 is a terminus license — meaning that once data is so licensed to cannot be returned to CC‑BY‑4.0 status. Moreover no other license (that I am aware of) will take OGL‑UK‑3.0 material so that therefore ends the path.
Continuing to promote licenses like the OGL‑UK‑3.0 or CDLA‑Permissive‑2.0 will necessarily fragment the open data space due to legal incompatibilities. This has relatively little to do with the operational merits of individual licenses and everything to do with how they play together. Moreover I cannot understand how legal siloing could be considered remotely desirable in aggregate. Sure there will be some edge cases where additional constraints are necessary and bespoke terms of use are indicated. But my focus is for general purpose public interest information and the CC‑BY‑4.0 is perfectly adequate. That license has also been endorsed for data by a number of bodies, including the European Commission and the German energy network regulator.
I would like to put together a position statement explaining these issues for the Open Knowledge Foundation to consider as a matter of license approval policy. But I am only willing to draft such a statement if it would be considered with an open mind. Please let me know, OKF.